Since resin materials having a high refractive index have higher processability than conventional glass materials, extensive studies are made to apply them to various applications such as eyeglass lenses, lenses for cameras and the like, lenses for optical disks, fθ lenses, optical elements of image display media, optical films, films, substrates, various types of optical filters, prisms, optical elements for communication, and the like.
In particular, resin materials having a fluorene structure are known to have a relatively high refractive index and a relatively low birefringence, and can also be expected to exhibit a high heat resistance, so such resin materials are variously being studied. Japanese Patent No. 4196326 discloses a low-birefringence polycarbonate resin having a 9,9′-diphenylfluorene structure and excelling in heat resistance and mechanical strength.
On the other hand, it is broadly known that naphthalene structures can make resin materials to have a higher refractive index than fluorene structures, and there is stated that a polyvinylnaphthalene has a refractive index of the d line of about 1.68, and is one of the polymers having a high refractive index among common polymers. A naphthalene skeleton has a high symmetry having its planar structure and a long conjugated system. Hence, introduction of a naphthalene structure into a polymer can also be expected to improve mechanical properties of a polymer.
From these backgrounds, resin materials having a naphthalene structure incorporated are variously being studied. For example, Japanese Patent Application Laid-Open No. H09-316190 discloses a novel aromatic diamine to become a raw material for an aromatic polyamide resin and a polyimide resin having a dimethylnaphthalene skeleton and excelling in mechanical strength, heat resistance and processability.
There is a spirobifluorene as a molecule having a structure by which a higher refractive index and a lower birefringence can be expected because of having a higher number of conjugated aromatic rings in a molecule thereof and a higher symmetry of the molecular structure than the conventional 9,9′-diphenylfluorene structure. Japanese Patent Application Laid-Open No. 2006-089585 discloses a resin having a heterocyclic structure having such a spiro carbon as a unit skeleton.
Optical elements are designed utilizing such resins.